Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as energy sources. Among these secondary batteries, lithium secondary batteries having high energy density and voltage, long lifespan and low self-discharge are commercially available and widely used.
Lithium secondary batteries generally use a carbon material as an anode active material. Also, the use of lithium metals, sulfur compounds and the like as the anode active material has been considered. Meanwhile, the lithium secondary batteries generally use lithium cobalt composite oxide (LiCoO2) as a cathode active material. Also, the use of lithium-manganese composite oxides such as LiMnO2 having a layered crystal structure, LiMn2O4 having a spinel crystal structure and lithium nickel composite oxide (LiNiO2) as the cathode active material has been considered.
LiCoO2 is currently used owing to superior physical properties such as excellent cycle life, but has disadvantages of low stability, high cost due to use of cobalt, which suffers from natural resource limitations, and limitations of mass-use as a power source for electric automobiles. LiNiO2 is unsuitable for practical application to mass-production at a reasonable cost due to many features associated with preparation thereof. Lithium manganese oxides such as LiMnO2 and LiMn2O4 have a disadvantage of short cycle life.
In recent years, methods of using lithium transition metal phosphate as a cathode active material have been researched. Lithium transition metal phosphate is largely divided into LixM2(PO4)3 having a NASICON structure and LiMPO4 having an olivine structure, and is found to exhibit superior high-temperature stability, as compared to conventional LiCoO2. To date, Li3V2(PO4)3 having a NASICON structure is well-known, and LiFePO4 and Li(Mn, Fe)PO4 are the most widely known olivine structure compounds.
Among olivine structure compounds, LiFePO4 has a high output voltage of ˜3.5 V and a high theoretical capacity of 170 mAh/g, as compared to lithium (Li), and exhibits superior high-temperature stability, as compared to cobalt (Co), and utilizes cheap Fe, thus being highly applicable as the cathode active material for lithium secondary batteries. However, such an olivine-type LiFePO4 has an operational efficiency of about 100%, thus making it difficult to control the operational efficiency of an anode.
In this regard, by imparting equivalent operational efficiency to a cathode and an anode in batteries, inefficient waste of the electrodes can be minimized. For example, in the case where an anode having efficiency of about 100% is used for a battery, the battery can exert 100% efficiency, while when a cathode having 100% efficiency and an anode having 90% efficiency are used for a battery, the battery can exert only 90% efficiency. As a result, 10% of the efficiency of the cathode is disadvantageously wasted.
For example, in the case of generally-used carbon-based anode active materials, about 10-20% irreversible capacity is generated upon initial charge/discharge including the first charge and its reversible capacity is only about 80 to 90%. Accordingly, when a material having an efficiency of 100% is used as a cathode active material, the electrode material is disadvantageously wasted in direct proportion to the irreversible capacity of about 10 to 20%. In addition, when an anode active material having relatively low efficiency is used, an amount of the anode active material should be increased, depending on a higher efficiency of a cathode, which disadvantageously entails an increase in manufacturing costs.
On the other hand, in order to impart 100% efficiency to a battery using a cathode having 100% efficiency, an anode having about 100% efficiency should be used. In this case, the selection range of an anode active material is disadvantageously narrowed.
However, to date, there is no technology suggesting a method for controlling efficiency of LiFePO4 as a cathode active material.
In addition, there is an increasing need for a breakthrough that can considerably improve electrical conductivity of LiFePO4 and solve Li+ diffusion problems thereof via improvement in initial IR drop and Li+ diffusion properties.
Furthermore, in a case where LiFePO4 is used as a cathode active material, an internal resistance of batteries disadvantageously increases due to low electrical conductivity thereof and a limitation on sufficient increase of energy density due to low density, as compared to common cathode active materials. Further, olivine crystal structures in which lithium is deintercalated are highly unstable, thus disadvantageously entailing blocking of passage of a region, where lithium on crystal surfaces is deintercalated, and delay of lithium intercalation/deintercalation rates.
In response to this, a decrease in size of olivine crystals to a nanometer-scale in order to shorten a movement distance of lithium ions and thus increase discharge capacity has been suggested (see. Japanese Patent Application Publication Nos. 2002-15735 and 2004-259470).
However, fabrication of electrodes using such an olivine particle with a fine diameter inevitably entails use of a large amount of binder, thus disadvantageously lengthening slurry mixing time and deteriorating process efficiency.
Accordingly, there is an increasing need for lithium iron phosphate such as LiFePO4 that exhibits superior electrical conductivity and density as well as process efficiency.